The accurate delivery of radiotherapy to a patient depends on a number of factors, including the accurate determination of the patient's current position, in terms of both their gross external position and the position of the internal structures that are to be irradiated or avoided. Some form of investigative x-ray apparatus is therefore a valuable part of a radiotherapy apparatus.
Given that the apparatus itself is capable of producing a beam of x-rays, it might be thought that this could be used as an investigative source. However, the therapeutic beam is typically at a high energy (in the MV range) and therefore the image contrast is poor and the dose delivered to the patient is relatively high. The poor contrast results from the attenuation coefficients that apply at higher energies as opposed to those that apply, at lower energies. At higher energies, the coefficients of bone and tissue are similar, thereby limiting the potential contrast that is obtainable.
It is therefore desirable to use a lower energy beam for investigative purposes. Beams with energies in the kV range can be detected more easily, apply a lower dose to the patient, and interact mainly via the photoelectric effect. The latter effect is dependent on atomic number, and the large difference between bone (20Ca) and water (1H and 8O) therefore allows a much better image contrast.
However, a separate source of kV x-rays presents various engineering difficulties. Such a source inherently adds additional cost and complexity to the apparatus. Further, spatial clearance requirements dictate that such sources view the patient along an axis that is offset by 90° from the therapeutic beam axis. Thus, as the therapeutic source is rotated around the patient, the diagnostic source is likewise rotated. These axes need to be aligned, and need to be kept in alignment.
It is therefore desirable to achieve a co-incident investigative kV source for a therapeutic MV source—a so-called “beams-eye-view” source. However, this is not a trivial step.
Galbraith (“Low-energy imaging with high-energy bremsstrahlung beams”, Medical Physics Vol. 16 No. 5, September/October 1989 pp 734-746) reported that simple replacement of the Tungsten or Copper target with a low-Z Carbon or Beryllium target allowed the production of a low-energy beam which could be used for diagnosis. Galbraith also noted that the electron beam will interact with the electron window to produce bremsstrahlung radiation which he was able to use for imaging. Accelerators typically operate by producing a high-energy beam of electrons; this is allowed to impinge on a target to produce x-rays. The electron beam moves from its vacuum enclosure to the atmosphere via an “electron window” in the enclosure, of Aluminium in Galbraith's case. Galbraith noted that in doing so, the beam produced x-radiation. Normally, this would be absorbed by the conventional treatment target, but without a target it is free for use in diagnosis.
Galbraith's suggestion of the electron window as a target also left the hypothetical patient being irradiated with the main part of the electron beam. Galbraith concluded that manufacturers should incorporate diagnostic modes in future accelerators to allow for modification in this direction, as the application of the method to standard accelerators “would in general be a difficult task”.
Flampouri et al. (“Optimisation of megavoltage beam and detector characteristics for portal imaging in radiotherapy”, PhD thesis, University of London, 2003) demonstrated the replacement of the conventional Tungsten or Copper target for an MV source with an aluminium target and the removal of the conventional flattening filter, to produce a low energy beam from the apparatus otherwise used to produce an MV beam suitable for imaging, including projection radiographs and CT imaging using the treatment machine.
Zheng et al (“Simple Beamline Modifications for High Performance Portal Imaging”, 8th International Workshop on Electronic Portal Imaging, Brighton, UK, 29th Jun. to 1 Jul. 2004) reported the replacement of the conventional Tungsten or Copper target for an MV source with a graphite or aluminium target and the removal of the conventional flattening filter, to produce a low energy beam from the apparatus otherwise used to produce an MV beam.
To allow for interchangeability of the target, however, the cassette carrying the standard and graphite or aluminium targets is located outside the vacuum enclosure, and therefore some distance from the source. Zheng does not discuss any interaction between the electron beam and the window, although he references Galbraith.